观点

大流行病:人类是罪魁祸首

生态系统遭到破坏,不仅对地球而言是坏消息,对人类健康而言,也是贻害无穷。2020年出现的2019冠状病毒病(Covid-19)大流行只是人畜共患疾病(动物传播给人类的疾病)扩散的一种表现。

约翰·维达尔(John Vidal)

记者、作家、《卫报》环境新闻前任编辑

1997年,我曾前往婆罗洲,调查一起森林火灾的情况,因火势失控,这场火灾在广袤的原始热带森林里燃烧数月。当时,一场超强厄尔尼诺事件引发了严重旱情,印度尼西亚、马来西亚,乃至更远的地方都笼罩在浓密的黄色烟雾之中。

生态系统和人类都伤痕累累。地球上远离尘嚣、自然资源丰富的部分原始森林燃烧着,成千上万的植物、鸟类和稀有动物(例如,猩猩)物种岌岌可危。阳光暗淡,气温下降,树木不开花,庄稼难生长,数百万人患上了严重的呼吸道疾病。

季风带来的降雨终于浇灭了大火,几个月后,数百英里之外、马来西亚吉隆坡以西一个名叫双溪尼帕的小镇附近,暴发了一种致死性神秘疾病。那里的人们原本在商业化的芒果和榴莲种植园中饲养着数万头猪。首先发病的是猪群,随后,许多人因不明原因的癫痫和头痛而纷纷倒下。为了阻止这种传染性极强的新发疾病蔓延,人们不得不屠杀了近百万头猪,但在此之前,已有105人死亡。

历时六年,疾病生态学家才将婆罗洲森林的毁损与马来西亚养猪场的疾病联系起来。2004年,一切终于真相大白:过去,某些种类的果蝠常常在婆罗洲森林深处开花和结果的树上觅食,但因1997年的大火而被迫寻找新的食物来源。

蝙蝠是病毒宿主

部分果蝠已经迁移到了双溪尼帕,那里的人曾经看到它们栖息在树上,将吃了一半的果子丢进树下的多个猪舍里。众所周知,蝙蝠是多种病毒的宿主,与非洲暴发的埃博拉和马尔堡病等致命疾病有关;科学家们发现,迁移到马来西亚的蝙蝠身上携带尼帕病毒,并通过水果和尿液将这种病毒传染给了猪群。

过去50年中,动物将数百种动物传染病或人畜共患疾病传染给了人类,尼帕病毒只是其中之一。人们愈发相信,许多疾病传染给人类的直接原因是人类对自然的破坏——如今有100万个物种濒临灭绝。

伦敦大学学院(UCL)生态学和生物多样性专业教授凯特·琼斯(Kate Jones)说:“我们对自然的破坏程度越深,暴发Covid-19之类可怕疾病的可能性就越大。”她认为,新疾病伴随着生物多样性的破坏而暴发,这种情况不容小觑。 

这些新疾病包括人类曾经遇到过的一些最致命的疾病,例如:源于非洲的艾滋病毒、埃博拉、拉沙热、马尔堡病和猴泡沫病毒;东南亚的尼帕病毒;拉丁美洲的美洲锥虫病、马丘波病毒和汉坦病毒;澳大利亚的亨德拉病毒;沙特阿拉伯的中东呼吸综合征;严重急性呼吸综合征(SARS)和2019冠状病毒病(Covid-19)。

有些疾病(例如,埃博拉)与毁林有关;还有些疾病(例如,莱姆病)发生在郊区向新开垦土地扩张的地方。人们认为,更多的疾病源于狩猎行为,或者与野生动物市场和动物的集约化养殖有关。

大规模毁坏森林

琼斯说:“生物多样性的丧失正在成为部分病毒出现的一个重要驱动因素。大规模毁林、栖息地退化和碎片化、农业集约化、我们的粮食系统、物种和植物贸易、人为的气候改变——所有这些都是导致生物多样性丧失的因素,也是导致新疾病出现的因素。如今,三分之二的新发感染和疾病源自野生动物。”

如今,三分之二的新发感染和疾病源自野生动物。

自然服务协会是一家总部设在美国的非营利组织,该组织的科学家与全球保护组织开展了合作。该协会的主席兼首席执行官肖恩·奥布莱恩(Sean O’Brien)坚持认为:“生物多样性的丧失或减少并非罪魁祸首,人类与生物多样性之间的相互作用才是。”

奥布莱恩解释说,集约农业,特别是砍伐森林以扩大农业规模,会增加人类与野生动物接触的频率,并使我们暴露在人类从未遇到过的疾病面前。“我们使自然界中永远不会天然相遇的野生动物聚到一起,在某个链上创造出若干奇怪的环节,导致疾病得以由一个物种通过另一个物种传染给人类,即使这种疾病可能无法直接传染人类。”

受干扰的生态系统

纽约州米尔布鲁克的卡里生态系统研究所高级科学家理查德·奥斯特菲尔德(Richard Ostfeld)研究了环境退化的地方出现莱姆病等疾病的情况。他说:“病原体在野生动物宿主中传播,有些有传播给人类的可能,但在未受干扰的自然生态系统中,这种情况极为罕见。然而,当人类造成自然栖息地的退化或破坏时,动物群落就会发生巨变。”

“一些最重要的人畜共患疾病宿主(啮齿动物,有时还有蝙蝠)常常因捕食者和竞争对手遭到驱逐而大量增加。生物多样性的丧失增加了啮齿动物或蝙蝠与人类之间的接触率,从而加剧了传染病威胁。”奥斯特菲尔德如是说。

卡洛斯·赞布拉纳-托雷利奥(Carlos Zambrana-Torrelio)是生态健康联盟负责生态保护和健康的助理副主席,也是玻利维亚国家植物标本馆的研究员,与联合国《生物多样性公约》一起研究了生物多样性与人类行为之间的联系。他指出:“目前的Covid-19大流行并非首个源自野生动物、致死率很高的流行病和大流行病。艾滋病毒就是从灵长类动物传染给人类的,汉坦病毒和玻利维亚的马丘波病毒等出血热是从啮齿动物传染给人类的。”

物种越多,疾病越少

生物多样性的丧失是增加了还是减少了传染给人类的病毒数量,这是一个很复杂的问题。从逻辑上看,生物多样性越丰富,动物之间传播的病原体和病毒就越多,因此,传染给人类的可能性就越大。然而,多项研究表明,物种越多,意味着疾病越少——丰富的生物多样性可以保护共同进化的物种。只有当自然系统受到干扰时,冠状病毒或埃博拉等病毒才会传播。

只有当自然系统受到干扰时,冠状病毒等病毒才会传播。

纽约州安嫩代尔的巴德学院疾病生态学家菲利西亚·基辛(Felicia Keesing)研究了世界各地生态系统中的12种疾病,包括西尼罗河热和莱姆病。在每一项研究中,她都发现,随着生物多样性的丧失,疾病会日益猖獗。

联合王国利物浦大学兽医传染病学主任埃里克·费夫尔(Eric Fèvre)说,在缺乏生物多样性的栖息地,物种聚集在一起,也会传播新疾病。“养殖动物往往是生物多样性丧失的最终产物。我们在挑选更好的牛、鸡或猪时,就创造了密集生活的动物种群,它们具有非常相似的遗传基因。这就带来了患病风险,因为如果这些数量巨大、基因相同的动物易受感染,那么疾病就会迅速传播。”他说。

这位专家的观点得到了加州大学戴维斯分校兽医学院共享健康研究所疾病动态中心的负责人——克里斯汀·克鲁德·约翰逊(Christine Kreuder Johnson)的支持。在一项为期四年的新研究中,她证明了,正是那些遭到人类猎杀且栖息地惨遭人类严重破坏的动物,携带对人类而言最危险的病毒。

约翰逊说:“结果,它们把病毒传给了我们。人类自身的行为既威胁着物种的生存,又加剧了溢出风险。不幸的是,多种因素的汇集,导致我们陷入了当前的困境。”

她说,我们打破了物种之间的天然屏障,破坏了生物多样性,让Covid-19趁虚而入,不仅如此,更多的病毒和病原体也可能有了可乘之机。

 

拓展阅读:

《失去光泽的小金蛙》,联合国教科文组织《信使》,2018年4-6月。

《人与自然:和谐相处》,联合国教科文组织《信使》,2009年6月。

 

订阅联合国教科文组织《信使》,阅读发人深省的时事文章,数字版免费。

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By John Vidal

 

In 1997, I went to Borneo to investigate fires which had been raging uncontrolled for months across a vast area of pristine tropical forest. An intense El Niño event had triggered a deep drought, and a thick yellow haze had settled over much of Indonesia, Malaysia, and beyond.

The ecological and human damage was immense. Some of the most undisturbed, nature-rich forest on earth was burning, and thousands of species of plants, birds, and rare animals like orangutans were at risk. The sunlight had dimmed, the temperature had dropped, trees were not flowering, crops barely grew, and millions of people were suffering from severe respiratory diseases.

Months after monsoon rains finally doused the fires, a mysterious, deadly disease broke out hundreds of miles away – near a town called Sungai Nipah, west of Kuala Lumpur,  Malaysia. Here, tens of thousands of pigs were being farmed among commercial mango and durian orchards. For no discernible reason, first the pigs, and then many humans, were struck down by seizures and headaches. To stop the new, highly infectious disease spreading, nearly a million pigs had to be destroyed – but not before 105 people had died.

It took six years for disease ecologists to link the destruction of the forest in Borneo with the  disease in the Malaysian pig farms. What had happened, it emerged in 2004, was that certain species of fruit bats which usually foraged on flowering and fruiting trees deep in the Borneo forests, had been forced by the 1997 fires to seek new food sources.

Bats as virus reservoirs

Some of these bats had gone to Sungai Nipah, where they had been observed roosting in the trees and dropping pieces of half-eaten fruit into the many pigpens below. Bats are well-known reservoirs of many viruses and – just as they have been linked to the emergence of deadly diseases like Ebola and Marburg in Africa – scientists found that bats arriving in Malaysia carried the Nipah virus, which they had passed to the pigs in the fruit and their urine.

Nipah is just one of many hundreds of animal-borne, or zoonotic diseases, which have jumped from animals to humans in the past fifty years. Increasingly, it is believed, many have done so as a direct result of the human devastation of nature – which now sees one million species threatened with extinction.

“The more we destroy nature, the more likely we are to see fearsome diseases like Covid-19 emerging,” says Kate Jones, professor of ecology and biodiversity at University College London (UCL). The coincidence of the new diseases with the destruction of biodiversity is highly significant, according to her.  

They include some of the most deadly diseases ever encountered by humans – like HIV, Ebola, Lassa fever, Marburg, and Simian foamy virus, all of which originated in Africa; Nipah virus in South East Asia; Chagas disease, Machupo and hantavirus in Latin America; Hendra in Australia; Middle East Respiratory Syndrome (MERS) in Saudi Arabia; as well as Severe Acute Respiratory Syndrome (SARS) and coronavirus disease (Covid-19), both of which first emerged in China.

Some, like Ebola, have been linked to deforestation; others, like Lyme disease, occur where suburban areas sprawl into newly-cleared land. Many more are believed to have been caused by hunting, or are associated with wildlife markets and the intensive farming of animals.

Deforestation on a massive scale

“Biodiversity loss is becoming a big driver in the emergence of some of these viruses. Large-scale deforestation, habitat degradation and fragmentation, agriculture intensification, our food system, trade in species and plants, anthropogenic climate change – all these are drivers of biodiversity loss and also drivers of new diseases. Two-thirds of emerging infections and diseases now come from wildlife,” Jones says.

Two-thirds of emerging infections and diseases now come from wildlife

“The loss or reduction of biodiversity is not the culprit. It is the interactions between humans and biodiversity,” insists Sean O’Brien, president and CEO of NatureServe, a non-profit headquartered in the United States, with scientists working with global conservation organizations.

Intensive farming – especially the clearing of forests to expand farming – can increase the frequency of contact between humans and wildlife and expose us to diseases never encountered before by humans, O’Brien explains. “We are bringing together wildlife that would never naturally encounter each other in nature, creating bizarre links in a chain that can allow a disease to jump from one species to humans via another species, even if that disease might not be able to get to us directly.” 

Disturbed ecosystems

“Pathogens circulate in wildlife hosts, and some have the potential to jump to humans, but they rarely do, in undisturbed, natural ecosystems. But when people degrade or destroy natural habitats, the animal community changes dramatically,” says Richard Ostfeld, Senior Scientist at the Cary Institute of Ecosystem Studies in Millbrook, New York, who researches how diseases like Lyme emerge in degraded places.

“Some of the most important zoonotic hosts – the rodents, and sometimes, bats – often increase in abundance as their predators and competitors are driven away. Biodiversity loss increases the contact rates between the rodents or bats and people, so increasing the threat of infectious diseases,” Ostfeld says.

Carlos Zambrana-Torrelio, Associate Vice President for Conservation and Health at EcoHealth Alliance, and a researcher at the Bolivian National Herbarium, works on the links between biodiversity and human actions with the United Nations’ Convention on Biological Diversity (CBD). “The present Covid-19 pandemic is not the first time we have seen these epidemics and pandemics coming from wildlife and causing very high mortality. HIV jumped from primates to humans, haemorrhagic fevers like hantavirus and Machupo virus in Bolivia jumped from rodents into humans,” he points out. 

More species, fewer diseases

Whether biodiversity loss increases the number of viruses passing to humans, or reduces them, is complex. Logically, the richer the biodiversity, the more pathogens and viruses will be circulating in animals,  and are therefore more likely to jump to humans. However, many studies show that more species mean less diseases – and that a rich biodiversity acts to protect species which evolve together. It is only when a natural system is disturbed that viruses like coronavirus or Ebola are passed on.

It is only when a natural system is disturbed that viruses like coronavirus are passed on

Felicia Keesing, a disease ecologist at Bard College in Annandale, New York, has studied twelve diseases, including West Nile fever and Lyme disease, in ecosystems around the world. In every study, she found that diseases became more prevalent as biodiversity was lost.

Species which are crammed together in biodiversity-poor habitats can also spread emerging diseases, says Eric Fèvre, Chair of Veterinary Infectious Diseases at the University of Liverpool in the United Kingdom. “Farmed animals are often the end product of a loss of biodiversity. As we select for better cows, chickens or pigs, we create populations of animals that often live in intensive conditions, but where the genetics are very similar. This creates risks for the emergence of diseases, because if these genetically-uniform large populations are susceptible, disease can spread very quickly,” he explains. 

The expert is backed by Christine Kreuder Johnson, who leads the EpiCenter for Disease Dynamics of the One Health Institute at the School of Veterinary Medicine, University of California, Davis. In a new four-year study, she shows how it is the animals that humans hunt, and whose habitats we destroy the most, which are the ones with the most viruses dangerous to humans. 

“The consequence is, they’re sharing their viruses with us. It is actions by humans which simultaneously threaten species survival and increase the risk of spillover. In an unfortunate convergence of many factors, this brings about the kind of mess we’re in now,” said Johnson.

By breaking down the natural barriers between species and destroying biodiversity, she argues, we have opened the door – not just to Covid-19, but potentially to many more viruses and pathogens.

John Vidal

Journalist, author, and former environment editor of The Guardian newspaper.

Restoring biodiversity, reviving life
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July-September 2021
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